1. Technical Field
The present invention relates to a motor driving device for controlling a motor, the motor driving device converting alternating current power from an alternating current power supply into direct current power, and then inverting the direct current power into alternating current power used as driving power for the motor. In particularly, the present invention relates to a motor driving device that can suppress generation of abnormal excessive voltage in a converter.
2. Description of Related Arts
In a machine tool system, a motor is provided for each drive shaft of a machine tool, and these motors are controlled and driven by a motor driving device. The motor driving device instructs each of the motors, on speed, torque or a rotor position of the motor, which are provided for driving the corresponding drive shafts of the machine tool. Thus, the motor driving device controls the motors.
The motor driving device has a converter and an inverter. The converter converts commercial three-phase alternating current power into direct current power. The inverter inverts the direct current power output by the convertor into alternating current power that has a desired frequency and that is used as driving power for the motor, or converts alternating current power regenerated by the motor into direct current power. The motor driving device controls speed, torque or rotor position of the motor connected on the alternating current side of the inverter.
The number of the provided inverters is the same as the number of motors. These inverters individually supply driving electric power to the motors provided for a plurality of drive shafts in a machine tool to control the motors. Meanwhile, in many cases, one converter is provided for a plurality of inverters in order to reduce the cost of the motor driving device or the occupied space.
When the motor driving device controls a speed of the motor to be lowered, the motor regenerates electric power. The regenerated electric power is returned to the side of the converter via the inverter. Thus, the inverter is configured as a semiconductor electric power converter, having the ability to not only invert direct current power into alternating current power, but also convert alternating current power into direct current power, in other words, capable of converting electric power from alternating current power to direct current power and vice versa.
Electric power regenerated by the motor at the time of control of decelerating the motor raises a voltage on the direct current output side of the converter. High regenerated electric power sharply raises a voltage on the direct current output side of the converter to generate an abnormal excessive voltage. As a result, the converter may be damaged.
In order to avoid generation of such an abnormal excessive voltage on the direct current output side of the converter, a regeneration resistor or a power supply regenerative circuit is additionally provided. The regeneration resistor is provided on the direct current output side of the converter. The regeneration resistor generates heat to consume the electric power corresponding to the raised amount of the voltage on the direct current output side. The power supply regenerative circuit returns regenerated electric power to the side of the alternating current power supply.
For example, as described in Japanese Patent Application Laid-Open Publication No. H07-95775, a inverter-protecting device is proposed. The inverter-protecting device calculates regenerated electric power level on the basis of an output voltage of the converter and a flowing direction of a direct current flowing into the inverter. If the regenerated electric power level exceeds a predetermined value, the inverter-protecting device makes electrical connection to the regeneration resistor so as to generate heat to consume the electric power corresponding to the raised amount of the direct current voltage. In this manner, the inverter-protecting device prevents the inverter from being damaged due to excessive voltage.
Depending on magnitude of motor inertia or a motor speed, an excessive current larger than a rated value is generated at the time of regeneration, so that the regeneration resistor may be damaged. For this reason, a regeneration-resistor-protecting circuit or the like is proposed, which shuts off a current flowing to the regeneration resistor at the time of generation of excessive voltage.
For example, as described in Japanese Patent No. 3511173, a regeneration-resistor-protecting mechanism is proposed. This mechanism shuts off a current flowing to the regeneration resistor from a capacitor connected between the output terminals of the converter, at the time of detecting an excessive current larger than a rated value of the regeneration resistor. At the same time, this mechanism also turns off an input control contactor that controls an alternating current input to the converter. Thereby, burnout of the regeneration resistor can be prevented.
As another example, as described in Japanese Patent No. 3368930, a servomotor regeneration processing circuit is proposed. This circuit calculates regenerated electric power on the basis of a current, a speed, torque and the like of a servomotor, calculates decelerating torque to decelerate the servomotor such that the regenerated electric power does not exceed allowable regenerated electric power. Thereby, it is possible to prevent the regeneration resistor, a transistor and the like from being burned out by excessive voltage.
In order to avoid generation of abnormal excessive voltage on the direct current output side of the converter, a regeneration resistor that generates heat to consume the electric power corresponding to a raised amount of a voltage on the direct current output side is additionally provided on the direct current output side of the converter, or a power supply regenerative circuit that returns regenerated electric power to the side of the alternating current power supply may be additionally provided. In this case, if the motor regenerates excessive electric power, it is necessary to select a regeneration resistor or a power supply regenerative circuit having a large capacity in accordance with the magnitude of such excessively regenerated electric power. Unfortunately, depending on such a large capacity, the motor driving device is increased in size, and the cost increased.
For example, according to Japanese Patent Application Laid-Open Publication No. H07-95775 described above, magnitude of electric power regenerated by the motor at the time of regeneration differs depending on specification and operation of the motor. Accordingly, if the motor regenerates excessive electric power, it is necessary to select a regeneration resistor or a power supply regenerative circuit having a large capacity in accordance with the magnitude of such excessively regenerated electric power. Unfortunately, depending on such a large capacity, the cost of the motor driving device is increased.
According to Japanese Patent Nos. 3511173 and 3368930 described above, the regeneration-resistor-protecting circuit that protects the regeneration resistor is additionally provided. Due to the additional regeneration-resistor-protecting circuit, the cost of the motor driving device is unfortunately increased. Particularly, according to Japanese Patent No. 3368930 described above, in addition to the regeneration resistor, the regeneration-resistor-protecting circuit or the like needs to be provided, further increasing the cost of the motor driving device.